A crystalline colloidal array (CCA) is a three dimensionally ordered lattice of self-assembled monodisperse colloidal particles, typically amorphous silica or a polymer latex, dispersed in an aqueous or non-aqueous medium. At high particle concentrations, long-range electrostatic interactions between particles result in a significant inter-particle repulsion, which yields the adoption of a minimum energy colloidal crystal structure with either body-centered cubic or face-centered cubic symmetry.
Crystalline colloidal arrays can be formed having lattice spacings comparable to the wavelengths of ultraviolet, visible and infrared radiation. It has long been recognized that an array comparable in period to the wavelength of electromagnetic waves can provide an analog, i.e., a “bandgap,” which can act as a filter for a particular wavelength. Bragg diffraction techniques have been used to examine CCAs with a view towards identifying their interparticle spacing, lattice parameters and phase transitions. Because CCAs can be fabricated to diffract electromagnetic radiation, including the visible spectrum, such arrays have potential applications as optical filters, switches, limiters and sensors. However, the low elastic modulus exhibited by a liquid dispersion results in weak shear, gravitational, electric field, or thermal forces having the propensity to disturb the crystalline order and is a severe drawback to the practical application of CCAs in photonic devices.
Recently, approaches to develop robust network matrices have been pioneered to stabilize both organic and inorganic arrays through an in situ polymerization of a monomer around the ordered arrays. Specifically, colloidal crystals arrays have been stabilized through encapsulation in hydrogel networks and have been referred to as polymerized crystalline colloidal arrays (PCCAs). However, the PCCAs contain at least 30 percent by volume of water, resulting in their fragility and propensity for significant changes in optical performance with water content.
To overcome the drawbacks of the hydrogel networks CCAs have been encapsulated in essentially water-free polymeric matrices. However, one motivation for developing a more robust system was to achieve varying types of tunability, i.e., controllable changes of the CCA lattice spacings responsive to specific environmental stimuli. Yet, the water-free PCCAs that have been formed to date have exhibited limited tuning capabilities. Specifically, prior art composite films composed of silica particles in an acrylate polymeric matrix have exhibited band stop tuning responsive to mechanical stress, though the diffraction wavelength shifts were limited to about 50 nm or less and the time for the films to return to the optical characteristics of their unloaded state after the cessation of stress was from two to four hours.
Accordingly, there exists a need in the art for robust composites which exhibit radiation diffracting properties, which are tunable to a significant degree responsive to applied stress and which return to their initial optical characteristics immediately upon the cessation of stress.